This patent application is associated with the materials cited in U.S. Pat. No. 5,312,790 granted on May 17, 1994, entitled "Novel Ceramic Ferroelectric Material", U.S. patent Ser. No. 08/207,446 filed Mar. 7, 1994, now U.S. Pat. No. 5,427,988, entitled "Novel Ceramic Ferroelectric Composite Material--BSTO-MgO", U.S. patent Ser. No. 08/207,447 filed Mar. 7, 1994, now U.S. Pat. No. 5,486,491, entitled "Novel Ceramic Ferroelectric Composite Material--BSTO-ZrO.sub.2 " and U.S. patent Ser. No. 08/215,877 filed Mar. 22, 1994, entitled "Novel Ceramic Ferroelectrics for Phased Array Antennas".
The need exists for the fabrication of ceramic materials having improved electronic properties which may be adjusted for a particular, intended use. The present invention deals with novel ceramic ferroelectric materials having ideal properties for use, for example, in multilayer capacitors, capacitor-varistors, in non-volatile computer memory or for use in phased array antenna systems.
Recently, in the multilayer ceramic capacitor (MLC) market, there is a growing requirement for miniaturization, large capacitance, and low cost. The ferroelectric materials are a replacement for other high dielectric constant materials currently under investigation. These include lead based relaxor materials such as PbMg.sub.1/3 Nb.sub.2/3 O.sub.3 etc. The main problem with these materials is the difficulty encountered in the processing procedures. The volatilization of lead and the presence of impurities creates a deleterious pyrochlore phase which drastically lowers the dielectric constants which can be achieved and increases loss tangents in the material. Other Barium Strontium Titanate and Barium Titanate materials which are currently in use do not possess dielectric constants in the same range as those presented in this invention.
Another application for these materials is a combined capacitor-varistor device. In general this would be used as a protective device used in parallel with electronic information processing circuits to protect against spurious voltage surges and voltage transients. The capacitive aspect of the device would guard against low-amplitude and high frequency transients which cause errors in signal processing or in stored signals. The varistor function of the device protects against high-amplitude voltage surges. The device combination is a replacement for a capacitor Zener diode combination which is often bulkier than the circuit it is protecting.
The fabrication of the capacitor-varistor device has been attempted previously by appropriately doping SrTiO.sub.3 to form grain boundary phases or layers which exhibit the desirable characteristics. (N. Yamaoka, M. Masuyama and M. Fukui, "SrTiO.sub.3 -Based Boundary Layers Capacitor Having Varistor Characteristics", Ceramic Bulletin, vol 62, pp. 698-700 (1983).). Also discrete layers of ZnO and Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 --Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 (dielectric constant of 27000 to 32000) have been fabricated (H. C. Ling and M. F. Yan, "A Monolithic Capacitor-Varistor Device--Properties and Microstructure, Ceramic Transactions, vol. 8, pp. 253-260 (1989). (Proceedings of the Symposium on Ceramic Dielectrics held in Indianapolis, Ind., Apr. 23-27, 1989)). The problem with the former scheme has been with the high loss tangents and high threshold voltages derived from the metallic additives and the semiconducting capacitor. The problem associated with the latter scheme, as mentioned previously, is with the processing of the lead based capacitor compounds and the deleterious inter-diffusion of species from the ZnO varistor and lead-based capacitor layers. The materials described within the present invention circumvent some of these problems by combining the device into one composite ceramic material which can be processed as a bulk ceramic or as a thin film (by pulsed laser deposition) or thick film multi-layer (tape-cast) laminate.
At low doping levels of zinc oxide the dielectric constant of BSTO-ZnO is increased without increasing the loss tangent or without producing varistor characteristics. Therefore, these compositions would exhibit high dielectric constants and high tunabilities would be useful in many low to moderate frequency antenna applications.
Barium Strontium Titanate (BaTiO.sub.3 --SrTiO.sub.3), also referred to herein as BSTO, has been known to be used for its high dielectric constant (approximately ranging from 200 to 6,000) in various electronic applications. This component acts as the capacitive part of the device. ZnO is added to BSTO and contributes to the varistor characteristics of the device. Although the employment of BSTO in electronic devices is known, nowhere in the technical arena of ceramic art has there been any suggestion of modifying BSTO, or combining BSTO with additives, in the manner described herein. Moreover, the specific BSTO combinations, which have enhanced electronic properties, are deemed novel.